The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles suspended in a colored dielectric solvent. This general type of display was first proposed in 1969. An EPD typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between the electrodes. At least one of the electrodes, typically on the viewing side, is transparent. For the passive type of EPDs, row and column electrodes on the top (the viewing side) and bottom plates respectively are needed to drive the displays. In contrast, an array of thin film transistors (TFT) on the bottom plate and a common, non-patterned transparent conductor plate on the top viewing substrate are required for the active type EPDs. An electrophoretic fluid composed of a colored dielectric solvent and charged pigment particles dispersed therein is enclosed between the two electrodes.
When a voltage difference is imposed between the two electrodes, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the plates, can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages. No backlight is needed in this type of reflective EPD displays.
A transmissive EPD is disclosed in U.S. Pat. No. 6,184,856 in which a backlight, color filters, and substrates with two transparent electrodes are used. The electrophoretic cells serve as a light valve. In the collected state, the particles are positioned to minimize the coverage of the horizontal area of the cell and allow the backlight to pass through the cell. In the distributed state, the particles are positioned to cover the horizontal area of the pixel and scatter or absorb the backlight. However, the backlight and color filter used in this device consume a great deal of power and are not desirable for hand-held devices such as PDAs (personal digital assistants) and e-books.
Besides the normal top/bottom electrode switching mode of EPDs, reflective “in-plane” switching EPDs have been disclosed in E. Kishi, et al., “5.1: development of In-Plane EPD”, Canon Research Center, SID 00 Digest, pages 24-27 (2000) and Sally A. Swanson, et al., “5.2: High Performance Electrophoretic Displays”, IBM Almaden Research Center, SID 00 Digest, pages 29-31, (2000). However, only monochrome in-plane switching EPDs are disclosed in these references. To prepare a multicolor display, either color filters or isolated color pixels or cell structures are needed for color separation and rendition. Color filter is typically expensive and not power-efficient. On the other hand, the preparation of isolated pixels or cells for color separation and rendering in the in-plane switching mode has not been taught previously.
EPDs of different pixel or cell structures have been reported in prior art, for example, the partition-type EPD (M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol ED 26, No. 8, pp 1148-1152 (1979)) and the microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026), and each of these has its own problems as noted below.
In a partition-type EPD, there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movements of the particles such as sedimentation. However, difficulties are encountered in the formation of the partitions, the process of filling the display with the fluid, enclosing the fluid in the display, and keeping the suspensions of different colors separated from each other.
The microencapsulated EPD has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a dispersion of charged pigment particles that visually contrast with the dielectric solvent. The microcapsules are typically prepared in an aqueous solution and, to achieve a useful contrast ratio, their mean particle size is relatively large (50-150 microns). The large microcapsule size results in a poor scratch resistance and a slow response time for a given voltage because a large gap between the two opposite electrodes is required for large capsules. Also, the hydrophilic shell of microcapsules prepared in an aqueous solution typically results in sensitivity to high moisture and temperature conditions. If the microcapsules are embedded in a large quantity of a polymer matrix to obviate these shortcomings, the use of the matrix results in an even slower response time and/or a lower contrast ratio. To improve the switching rate, a charge-controlling agent is often needed in this type of EPDs. However, the microencapsulation process in an aqueous solution imposes a limitation on the type of charge-controlling agents that can be used. Other drawbacks associated with the microcapsule system include poor resolution and poor addressability for color applications.
An improved EPD technology was recently disclosed in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 (corresponding to WO01/67170), U.S. Ser. No. 09/759,212, filed on Jan. 11, 2001, U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 (corresponding to WO02/01281) and U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001, all of which are incorporated herein by reference. The improved EPD comprises closed isolated cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent. The electrophoretic fluid is isolated and sealed in each microcup.
The microcup structure, in fact, enables a format flexible, efficient roll-to-roll continuous manufacturing process for the preparation of EPDs. The displays can be prepared on a continuous web of conductor film such as ITO/PET by, for example, (1) coating a radiation curable composition onto the ITO/PET film, (2) making the microcup structure by a microembossing or photolithographic method, (3) filling the electrophoretic fluid and sealing the microcups, (4) laminating the sealed microcups with the other conductor film, and (5) slicing and cutting the display to a desirable size or format for assembling.
One advantage of this EPD design is that the microcup wall is in fact a built-in spacer to keep the top and bottom substrates apart at a fixed distance. The mechanical properties and structural integrity of microcup displays are significantly better than any prior art displays including those manufactured by using spacer particles. In addition, displays involving microcups have desirable mechanical properties including reliable display performance when the display is bent, rolled, or under compression pressure from, for example, a touch screen application. The use of the microcup technology also eliminates the need of an edge seal adhesive which would limit and predefine the size of the display panel and confine the display fluid inside a predefined area. The display fluid in a conventional display prepared by the edge sealing adhesive method will leak out completely if the display is cut in any way, or if a hole is drilled through the display. The damaged display will be no longer functional. In contrast, the display fluid in the display prepared by the microcup technology is enclosed and isolated in each cell. The microcup display may be cut to almost any dimension without the risk of damaging the display performance due to the loss of display fluids in the active areas. In other words, the microcup structure enables a format flexible display manufacturing process, wherein the process produces a continuous output of displays in a large sheet format which can be sliced and diced to any desired format. The isolated microcup or cell structure is particularly important, when cells are filled with fluids of different specific properties such as colors and switching rates. Without the microcup structure, it will be very difficult to prevent the fluids in adjacent areas from intermixing, or being subject to cross-talk during operation.
Multi-color displays may thus be manufactured by using a spatially adjacent array of small pixels formed of microcups filled with dyes of different colors (e.g., red, green or blue). However there is a major deficiency in this type of system with the traditional top/bottom electrode switching mode. The white light reflected from the “turned-off” colored pixels greatly reduces the color saturation of the “turned-on” colors. More details in this regard are given in the following “Detailed Description” section.
While this deficiency may be remedied by an overlaid shutter device such as a polymer dispersed liquid crystal to switch each pixel to the black color, the disadvantage of this approach is the high cost of the overlaid device and the complicated driving circuit design.
Thus, there is still a need for an EPD with improved properties that can also be prepared in an efficient manner.